Piezoceramic materials have the property of becoming electrically charged when impinged upon by mechanical forces, i.e. in particular under mechanical compression or tension. On the other hand, the result of an electric field applied to the piezoceramic material is that the material is mechanically distorted, i.e. expands or contracts.
These latter effects are utilized in actuators in order to perform positioning motions.
As a result of the construction of the actuator from a stack of piezoceramic films having a corresponding number of internal electrodes, a high electric field strength can be achieved within the piezoceramic films even with a limited electrical operating voltage, since in the case of two electrode groups, the operating voltage is present between each two adjacent internal electrodes.
Contacting of the internal electrodes can present practical difficulties. In conventional actuators, side regions of the stack that are separated from one another are metal-coated in such a way that the one coating is electrically connected to the internal electrodes of the one group, and the other coating is electrically connected to the internal electrodes of the other group.
Upon operation of the actuators, these metal coatings are exposed to considerable mechanical stresses when the actuator expands or contracts in accordance with the particular operating voltage. Large alternating stresses can occur in this context if the operating voltage is frequently switched on and off or is switched over in terms of its polarity.
These mechanical stresses in the metal coatings can result in cracks in the coating, with the consequence that a variable number of internal electrodes can no longer be connected to the operating voltage source and the adjacent piezoceramic films cannot, or essentially cannot, contribute further to the work of the actuator.
It is therefore proposed in German Patent 196 48 545 A1 to cover the aforesaid metal coatings with a further electrically conductive layer that is mechanically particularly flexible, in order to keep the fragments of the aforesaid coating which in German Patent 196 48 545 A1 are also referred to as the “base metallization”—continuously in electrically conductive contact with one another. This additional coating can, for example, take the form of a knitted or braided wire structure, or that of a metal foam or corrugated sheet.